Optical filter and sensor system

ABSTRACT

An optical filter having a passband at least partially overlapping with a wavelength range of 800 nm to 1100 nm is provided. The optical filter includes a filter stack formed of hydrogenated silicon layers and lower-refractive index layers stacked in alternation. The hydrogenated silicon layers each have a refractive index of greater than 3 over the wavelength range of 800 nm to 1100 nm and an extinction coefficient of less than 0.0005 over the wavelength range of 800 nm to 1100 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/099,180, filed Apr. 14, 2016, which is a continuation of U.S. patentapplication Ser. No. 13/943,596, filed Jul. 16, 2013 (now U.S. Pat. No.9,354,369), which claims priority from U.S. Provisional PatentApplication No. 61/672,164, filed on Jul. 16, 2012, the contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to optical filters and to sensor systemscomprising optical filters. More particularly, the present inventionrelates to optical filters including hydrogenated silicon layers and tosensor systems comprising such optical filters.

BACKGROUND OF THE INVENTION

In a typical gesture-recognition system, a light source emitsnear-infrared light towards a user. A three-dimensional (3D) imagesensor detects the emitted light that is reflected by the user toprovide a 3D image of the user. A processing system then analyzes the 3Dimage to recognize a gesture made by the user.

An optical filter, more specifically, a bandpass filter, is used totransmit the emitted light to the 3D image sensor, while substantiallyblocking ambient light. In other words, the optical filter serves toscreen out ambient light. Therefore, an optical filter having a narrowpassband in the near-infrared wavelength range, i.e., 800 nm to 1100 nm,is required. Furthermore, the optical filter must have a hightransmittance level within the passband and a high blocking leveloutside of the passband.

Conventionally, the optical filter includes a filter stack and ablocking stack, coated on opposite surfaces of a substrate. Each of thestacks is formed of high-refractive-index layers andlow-refractive-index layers stacked in alternation. Different oxidesare, generally, used for the high-refractive-index layers and thelow-refractive-index layers, such as TiO₂, Nb₂O₅, Ta₂O₅, SiO₂, andmixtures thereof. For example, some conventional optical filters includea TiO₂/SiO₂ filter stack and a Ta₂O₅/SiO₂ blocking stack, in which thehigh-refractive index layers are composed of TiO₂ or Ta₂O₅,respectively, and the low-refractive-index layers are composed of SiO₂.

In a first conventional optical filter designed to transmit light in awavelength range of 829 nm to 859 nm over an incidence angle range of 0°to 30°, the filter stack includes 71 layers, the blocking stack includes140 layers, and the total coating thickness is about 24 μm. Transmissionspectra 100 and 101 at incidence angles of 0° and 30°, respectively, forthis optical filter are plotted in FIG. 1. In a second conventionaloptical filter designed to transmit light at a wavelength of 825 nm overan incidence angle range of 0° to 20°, the filter stack includes 43layers, the blocking stack includes 82 layers, and the total coatingthickness is about 14 μm. Transmission spectra 200 and 201 at incidenceangles of 0° and 20°, respectively, for this optical filter are plottedin FIG. 2. In a third conventional optical filter designed to transmitlight in a wavelength range of 845 nm to 865 nm over an incidence anglerange of 0° to 24°, the filter stack includes 77 layers, the blockingstack includes 148 layers, and the total coating thickness is about 26μm. Transmission spectra 300 and 301 at incidence angles of 0° and 24°,respectively, for this optical filter are plotted in FIG. 3.

With reference to FIGS. 1-3, the first, second, and third conventionaloptical filters, generally, have a high transmittance level within thepassband and a high blocking level outside of the passband. However, thecenter wavelength of the passband undergoes a relatively large shiftwith change in incidence angle. Consequently, the passband must berelatively wide to accept light over the required incidence angle range,increasing the amount of ambient light that is transmitted and reducingthe signal-to-noise ratio of systems incorporating these conventionaloptical filters. Furthermore, the large number of layers in the filterstacks and blocking stacks increases the expense and coating timeinvolved in fabricating these conventional optical filters. The largetotal coating thickness also makes these conventional optical filtersdifficult to pattern, e.g., by photolithography.

To enhance the performance of the optical filter in thegesture-recognition system, it would be desirable to reduce the numberof layers, the total coating thickness, and the center-wavelength shiftwith change in incidence angle. One approach is to use a material havinga higher refractive index than conventional oxides over the wavelengthrange of 800 nm to 1100 nm for the high-refractive-index layers. Inaddition to a higher refractive index, the material must have also havea low extinction coefficient over the wavelength range of 800 nm to 1100nm in order to provide a high transmittance level within the passband.

The use of hydrogenated silicon (Si:H) for high-refractive-index layersin optical filters is disclosed by Lairson, et al. in an articleentitled “Reduced Angle-Shift Infrared Bandpass Filter Coatings”(Proceedings of the SPIE, 2007, Vol. 6545, pp. 65451C-1-65451C-5), andby Gibbons, et al. in an article entitled “Development andImplementation of a Hydrogenated a-Si Reactive Sputter DepositionProcess” (Proceedings of the Annual Technical Conference, Society ofVacuum Coaters, 2007, Vol. 50, pp. 327-330). Lairson, et al. disclose ahydrogenated silicon material having a refractive index of 3.2 at awavelength of 1500 nm and an extinction coefficient of less than 0.001at wavelengths of greater than 1000 nm. Gibbons, et al. disclose ahydrogenated silicon material, produced by alternating current (AC)sputtering, having a refractive index of 3.2 at a wavelength of 830 nmand an extinction coefficient of 0.0005 at a wavelength of 830 nm.Unfortunately, these hydrogenated silicon materials do not have asuitably low extinction coefficient over the wavelength range of 800 nmto 1100 nm.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to an optical filter having apassband at least partially overlapping with a wavelength range of 800nm to 1100 nm, comprising: a filter stack including: a plurality ofhydrogenated silicon layers each having a refractive index of greaterthan 3 over the wavelength range of 800 nm to 1100 nm and an extinctioncoefficient of less than 0.0005 over the wavelength range of 800 nm to1100 nm; and a plurality of lower-refractive-index layers each having arefractive index of less than 3 over the wavelength range of 800 nm to1100 nm, stacked in alternation with the plurality of hydrogenatedsilicon layers.

The present invention also relates to a sensor system comprising: alight source for emitting light at an emission wavelength in awavelength range of 800 nm to 1100 nm; an optical filter having apassband including the emission wavelength and at least partiallyoverlapping with the wavelength range of 800 nm to 1100 nm, disposed toreceive the emitted light, for transmitting the emitted light whilesubstantially blocking ambient light, comprising: a filter stackincluding: a plurality of hydrogenated silicon layers each having arefractive index of greater than 3 over the wavelength range of 800 nmto 1100 nm and an extinction coefficient of less than 0.0005 over thewavelength range of 800 nm to 1100 nm; and a plurality oflower-refractive-index layers each having a refractive index of lessthan 3 over the wavelength range of 800 nm to 1100 nm, stacked inalternation with the plurality of hydrogenated silicon layers; and asensor, disposed to receive the emitted light after transmission by theoptical filter, for detecting the emitted light.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in greater detail with referenceto the accompanying drawings wherein:

FIG. 1 is a plot of transmittance spectra at incidence angles of 0° and30° for a first conventional optical filter;

FIG. 2 is a plot of transmittance spectra at incidence angles of 0° and20° for a second conventional optical filter;

FIG. 3 is a plot of transmittance spectra at incidence angles of 0° and24° for a third conventional optical filter;

FIG. 4 is a schematic illustration of a sputter-deposition system;

FIG. 5A is a plot of transmittance spectra for 1500-nm-thick siliconlayers deposited in the presence and absence of hydrogen;

FIG. 5B is a plot of the absorption-edge wavelength at a transmittancelevel of 50% against hydrogen flow rate for hydrogenated silicon (Si:H)layers before and after an annealing step;

FIG. 5C is a plot of refractive index at wavelengths of 800 nm to 1120nm against hydrogen flow rate for hydrogenated silicon layers;

FIG. 5D is a plot of absorption coefficient at wavelengths of 800 nm to880 nm against hydrogen flow rate for hydrogenated silicon layers;

FIG. 6 is a schematic illustration of a cross-section of an opticalfilter according to the present invention;

FIG. 7A is a table comparing properties of the first conventionaloptical filter of FIG. 1 and a first exemplary optical filter accordingto the present invention;

FIG. 7B is a table listing layer numbers, materials, and thicknesses forthe antireflective (AR) coating of the first exemplary optical filter;

FIG. 7C is a table listing layer numbers, materials, and thicknesses forthe filter stack of the first exemplary optical filter;

FIG. 7D is a plot of transmittance spectra at incidence angles of 0° and30° for the first exemplary optical filter;

FIG. 7E is a plot of transmittance spectra at incidence angles of 0° and30° for an optical filter analogous to the first exemplary opticalfilter, but including an Si/SiO₂ filter stack;

FIG. 8A is a table comparing properties of the second conventionaloptical filter of FIG. 2 and a second exemplary optical filter accordingto the present invention;

FIG. 8B is a table listing layer numbers, materials, and thicknesses forthe filter stack of the second exemplary optical filter;

FIG. 8C is a plot of transmittance spectra at incidence angles of 0° and20° for the second exemplary optical filter;

FIG. 8D is a plot of transmittance spectra at incidence angles of 0° and20° for an optical filter analogous to the second exemplary opticalfilter, but including an Si/SiO₂ filter stack;

FIG. 9A is a table listing layer numbers, materials, and thicknesses forthe filter stack of a third exemplary optical filter according to thepresent invention;

FIG. 9B is a plot of transmittance spectra at incidence angles of 0° and40° for the third exemplary optical filter; and

FIG. 10 is a block diagram of a sensor system according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an optical filter including hydrogenatedsilicon (Si:H) layers, which is particularly suitable for use in asensor system, such as a proximity sensor system, a three-dimensional(3D) imaging system, or a gesture-recognition system.

The optical filter of the present invention uses an improvedhydrogenated silicon material, which has both a high refractive indexand a low absorption coefficient over a wavelength range of 800 nm to1100 nm, i.e., in the near-infrared wavelength range. Typically, thehydrogenated silicon material is amorphous. The hydrogenated siliconmaterial is, preferably, produced by pulsed direct current (DC)sputtering. A sputter-deposition system suitable for producing thehydrogenated silicon material is described in U.S. Pat. No. 8,163,144 toTilsch, et al., issued on Apr. 24, 2012, which is incorporated herein byreference.

With reference to FIG. 4, a typical sputter-deposition system 400 usedto produce the hydrogenated silicon material includes a vacuum chamber410, a substrate 420, a cathode 430, a cathode power supply 440, ananode 450, a plasma activation source (PAS) 460, and a PAS power supply470. The cathode 430 is powered by the cathode power supply 440, whichis a pulsed DC power supply. The PAS 460 is powered by the PAS powersupply 470, which is a radio frequency (RF) power supply.

The cathode 430 includes a silicon target 431, which is sputtered in thepresence of hydrogen (H₂), as well as an inert gas such as argon, todeposit the hydrogenated silicon material as a layer on the substrate420. The inert gas is introduced into the vacuum chamber 410 through theanode 450 and the PAS 460. Alternatively, the walls of the vacuumchamber 410 may serve as the anode, and the inert gas may be introducedat a different location.

Hydrogen is introduced into the vacuum chamber 410 through the PAS 460,which serves to activate the hydrogen. Activated hydrogen is morechemically reactive and is, therefore, more likely to create Si—H bonds,which are thought to be responsible for the optical properties of thehydrogenated silicon material. The PAS 460 is located very close to thecathode 430, allowing the PAS plasma and the cathode plasma to overlap.Both atomic and molecular species of activated hydrogen are believed tobe present in the plasmas. The use of the PAS 460 allows thehydrogenated silicon layer to be deposited at a relatively highdeposition rate with a relatively low hydrogen content. Typically, thehydrogenated silicon layer is deposited at a deposition rate of 0.05nm/s to 1.2 nm/s, preferably, at a deposition rate of about 0.8 nm/s.Alternatively, the cathode plasma alone may be used for hydrogenactivation.

The optical properties of the hydrogenated silicon material dependprimarily on the hydrogen content in the vacuum chamber 410 and,therefore, on the hydrogen flow rate. However, they are also influencedby other parameters, such as the flow rate of the inert gas, the PASpower level, the cathode power level, and the deposition rate.

FIG. 5A shows transmission spectra 500 and 501 for 1500-nm-thick siliconlayers deposited in the presence of hydrogen, at a hydrogen flow rate of139 sccm, and in the absence of hydrogen, respectively. The siliconlayer deposited in the presence of hydrogen, i.e., the hydrogenatedsilicon layer, has a significantly higher transmittance level over thewavelength range of 800 nm to 1100 nm.

FIG. 5B shows curves 510 and 511 of the absorption-edge wavelength at atransmittance level of 50% against hydrogen flow rate for hydrogenatedsilicon layers before and after an annealing step, respectively. For theas-deposited hydrogenated silicon layers, the absorption-edge wavelengthdecreases with increasing hydrogen flow rate. Generally, theabsorption-edge wavelength varies approximately logarithmically withhydrogen flow rate. The absorption-edge wavelength is decreased furtherby the annealing step, which was carried out at a temperature of about300° C. for about 60 minutes. Typically, when an optional post-coatingannealing step is performed, the hydrogenated silicon layers areannealed at a temperature of up to 350° C. for up to 120 minutes,preferably, at a temperature of 250° C. to 350° C. for 30 to 90 minutes.In some instances, more than one annealing step may be performed.

Thus, the absorption-edge wavelength of the hydrogenated siliconmaterial can be tuned by adjusting the hydrogen flow rate and,optionally, by annealing. Likewise, the refractive index and theabsorption coefficient of the hydrogenated silicon material can also canbe tuned by adjusting the hydrogen flow rate and, optionally, byannealing. Typically, the hydrogenated silicon layers are deposited witha hydrogen flow rate of greater than 80 sccm, preferably, a hydrogenflow rate of about 80 sccm. However, it should be noted that thehydrogen content associated with this flow rate will depend on thepumping speed of the vacuum system.

FIG. 5C shows a plot of refractive index at wavelengths of 800 nm to1120 nm against hydrogen flow rate for as-deposited hydrogenated siliconlayers. The refractive index decreases with increasing hydrogen flowrate. Generally, the refractive index varies approximately linearly withhydrogen flow rate. In particular, the refractive index of thehydrogenated silicon layer produced at a hydrogen flow rate of 80 sccmis greater than 3.55 over the wavelength range of 800 nm to 1120 nm.

FIG. 5D shows a plot of absorption coefficient at wavelengths of 800 nmto 880 nm against hydrogen flow rate for as-deposited hydrogenatedsilicon layers (the absorption coefficient is less than 0.0001 atwavelengths of 920 nm to 1120 nm). The absorption coefficient decreaseswith increasing hydrogen flow rate. Generally, the absorptioncoefficient varies approximately exponentially with hydrogen flow rate.In particular, the absorption coefficient of the hydrogenated siliconlayer produced at a hydrogen flow rate of 80 sccm is less than 0.0004over the wavelength range of 800 nm to 1120 nm.

The improved hydrogenated silicon material, tuned to have suitableoptical properties, is used in the optical filter of the presentinvention. With reference to FIG. 6, the optical filter 600 includes afilter stack 610, which is disposed on a first surface of a substrate620. In most instances, the substrate 620 is a free-standing substrate,typically, a glass substrate, e.g., a borofloat glass substrate.Alternatively, the substrate 620 may be a sensor or another device. Whenthe substrate 620 is a free-standing substrate, an antireflective (AR)coating 630 is often disposed on a second surface of the substrate 620opposite the first surface. Typically, the AR coating 630 is amultilayer interference coating, e.g., a Ta₂O₅/SiO₂ coating. Alsotypically, the AR coating 630 has a thickness of 0.1 μm to 1 μm.

The filter stack 610 includes a plurality of hydrogenated silicon layers611, which serve as higher-refractive-index layers, and a plurality oflower-refractive-index layers 612 stacked in alternation. Usually, thefilter stack 610 consists of a plurality of hydrogenated silicon layers611 and a plurality of lower-refractive-index layers 612 stacked in asequence of (H/L)_(n), (H/L)_(n)H, or L(H/L)_(n). Typically, the filterstack 610 includes a total of 10 to 100 layers, i.e., 5≦n≦50. Alsotypically, the hydrogenated silicon layers 611 and thelower-refractive-index layers 612 each have a thickness of 3 nm to 300nm, and the filter stack 610 has a thickness of 1 μm to 10 μm.Generally, the layer numbers and thicknesses are selected according to aparticular optical design. Preferably, the optical filter 600 has atotal coating thickness, i.e., the thickness of the filter stack 610 andthe AR coating 630, of less than 10 μm.

The hydrogenated silicon layers 611 are composed of the improvedhydrogenated silicon material tuned to have a refractive index ofgreater than 3 and an extinction coefficient of less than 0.0005 overthe wavelength range of 800 nm to 1100 nm. Preferably, the hydrogenatedsilicon material has a refractive index of greater than 3.5 over thewavelength range of 800 nm to 1100 nm, e.g., a refractive index ofgreater than 3.64, i.e., about 3.6, at a wavelength of 830 nm. A higherrefractive index is usually desirable. However, generally, thehydrogenated silicon material has a refractive index of less than 4.5over the wavelength range of 800 nm to 1100 nm.

Preferably, the hydrogenated silicon material has an extinctioncoefficient of less than 0.0004 over the wavelength range of 800 nm to1100 nm, more preferably, an extinction coefficient of less than 0.0003over the wavelength range of 800 nm to 1100 nm. Typically, thehydrogenated silicon material has an extinction coefficient of greaterthan 0.01 at wavelengths of less than 600 nm, preferably, an extinctioncoefficient of greater than 0.05 at wavelengths of less than 650 nm.Because the hydrogenated silicon material is relatively stronglyabsorbing at wavelengths of less than 600 nm, an additional blockingstack is not necessary in the optical filter 600.

The lower-refractive-index layers 612 are composed of alower-refractive-index material having a refractive index lower thanthat of the hydrogenated silicon layers 611 over the wavelength range of800 nm to 1100 nm. Typically, the lower-refractive-index material has arefractive index of less than 3 over the wavelength range of 800 nm to1100 nm. Preferably, the lower-refractive-index material has arefractive index of less than 2.5 over the wavelength range of 800 nm to1100 nm, more preferably, a refractive index of less than 2 over thewavelength range of 800 nm to 1100 nm.

A lower refractive index is usually desirable for thelower-refractive-index layers 612 to increase the width of the blockingwavelength range, i.e., the stopband, of the optical filter 600,allowing the same blocking level to be achieved with fewer layers in thefilter stack 610. However, in some instances, a somewhat higherrefractive index that is still lower than that of hydrogenated siliconlayers 611 may be desirable to reduce the center-wavelength shift withchange in incidence angle, i.e., angle shift, of the optical filter 600.

In most instances, the lower-refractive-index material is a dielectricmaterial, typically, an oxide. Suitable lower-refractive-index materialsinclude silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), titanium dioxide(TiO₂), niobium pentoxide (Nb₂O₅), tantalum pentoxide (Ta₂O₅), andmixtures thereof, i.e., mixed oxides.

The optical filter 600 may be fabricated by using a sputtering process.Typically, the substrate 620 is provided in the vacuum chamber of asputter-deposition system similar to that illustrated in FIG. 4. Thehydrogenated silicon layers 611 and the lower-refractive-index layers612 are then sequentially deposited on the first surface of thesubstrate 620 to form the filter stack 610 as a multilayer coating.Typically, the hydrogenated silicon layers 611 are deposited bypulsed-DC sputtering of a silicon target in the presence of hydrogen, asdescribed heretofore. Also typically, the lower-refractive-index layers612 are deposited by pulsed-DC sputtering of one or more suitable metaltargets, e.g., a silicon target, an aluminum target, a titanium target,a niobium target, and/or a tantalum target, in the presence of oxygen.The AR coating 630 is deposited on the second surface of the substrate620 in a similar fashion. It should be noted that the order of formingthe filter stack 610 and the AR coating 630 is usually unimportant.

The optical filter 600 is an interference filter having a passband atleast partially overlapping with the wavelength range of 800 nm to 1100nm. The passband may include the entire wavelength range of 800 nm to1100 nm or, more typically, only a part of the wavelength range. Thepassband may be restricted to part or all of the wavelength range of 800nm to 1100 nm, or may extend beyond the wavelength range. Preferably,the optical filter 600 has a transmittance level, within the passband,of greater than 90% over the wavelength range of 800 nm to 1100 nm.

The optical filter 600 provides blocking outside of the passband, i.e.,a stopband on one or both sides of the passband, typically, over awavelength range of 400 nm to 1100 nm, preferably, over a wavelengthrange of 300 nm to 1100 nm. Preferably, the optical filter 600 has ablocking level, outside of the passband, of greater than OD2 over thewavelength range of 400 nm to 1100 nm, more preferably, a blocking levelof greater than OD3 over the wavelength range of 300 nm to 1100 nm.

In some instances, the optical filter 600 is a long-wavelength-pass edgefilter, and the passband has an edge wavelength in the wavelength rangeof 800 nm to 1100 nm. However, in most instances, the optical filter 600is a bandpass filter, preferably, a narrow bandpass filter. Typically,the passband has a center wavelength in the wavelength range of 800 nmto 1100 nm. Preferably, the passband has a full width at half maximum(FWHM) of less than 50 nm. Often, the entire passband is within thewavelength range of 800 nm to 1100 nm.

Generally, the optical filter 600 has a low center-wavelength shift withchange in incidence angle. Preferably, the center wavelength of thepassband shifts by less than 20 nm in magnitude with a change inincidence angle from 0° to 30°. Accordingly, the optical filter 600 hasa wide incidence-angle acceptance range.

The optical filter 600 may have a variety of optical designs. Ingeneral, the optical design of the optical filter 600 is optimized for aparticular passband by selecting suitable layer numbers, materials,and/or thicknesses for the filter stack 610. Some exemplary opticalfilters, described hereafter, include an Si:H/SiO₂ filter stack and aTa₂O₅/SiO₂ AR coating, coated on opposite surfaces of a borofloat glasssubstrate.

With reference to FIG. 7, a first exemplary optical filter is a narrowbandpass filter that is designed to transmit light in a wavelength rangeof 829 nm to 859 nm over an incidence angle range of 0° to 30°. Thefirst exemplary optical filter of FIG. 7 is comparable to the firstconventional optical filter of FIG. 1, and some properties of theoptical filters are compared in FIG. 7A. Design data, i.e., layernumbers (from substrate to air), materials, and thicknesses, for the ARcoating and the filter stack of the first exemplary filter are listed inFIGS. 7B and 7C, respectively. The filter stack includes 48 layers, theAR coating includes 5 layers, and the total coating thickness is about5.7 μm.

Transmission spectra 700 and 701 at incidence angles of 0° and 30°,respectively, for the first exemplary optical filter are plotted in FIG.7D. The first exemplary optical filter has a transmittance level, withinthe passband, of greater than 90%, and a blocking level, outside of thepassband, of greater than OD3 over a wavelength range of 450 nm to 1050nm. The passband has a center wavelength of about 850 nm and a FWHM ofabout 46.5 nm at an incidence angle of 0°. With change in incidenceangle from 0° to 30°, the center wavelength of the passband shifts byabout −12.2 nm.

Advantageously, the first exemplary optical filter of FIG. 7 includesfewer layers and has a smaller total coating thickness than the firstconventional optical filter of FIG. 1. In particular, the total coatingthickness of the first exemplary optical filter is about one quarter ofthe total coating thickness of the first conventional optical filter.Therefore, the first exemplary optical filter is less expensive tofabricate and is easier to pattern. Also advantageously, the firstexemplary optical filter has a lower center-wavelength shift with changein incidence angle. Therefore, the passband of the first exemplaryoptical filter can be significantly narrower while accepting light overthe same incidence angle range, improving the signal-to-noise ratio ofsystems incorporating the first exemplary optical filter.

The first exemplary optical filter may also be compared to an analogousoptical filter including an Si/SiO₂ filter stack, i.e., a filter stackincluding non-hydrogenated silicon layers, instead of an Si:H/SiO₂filter stack. Transmission spectra 710 and 711 at incidence angles of 0°and 30°, respectively, for this optical filter are plotted in FIG. 7E.The transmittance level within the passband of this optical filter istoo low to be useful.

With reference to FIG. 8, a second exemplary optical filter is anarrower bandpass filter that is designed to transmit light at awavelength of 825 nm over an incidence angle range of 0° to 20°. Thesecond exemplary optical filter of FIG. 8 is comparable to the secondconventional optical filter of FIG. 2, and some properties of theoptical filters are compared in FIG. 8A. Design data for the AR coatingof the second exemplary optical filter, which is the same as that of thefirst exemplary optical filter, are listed in FIG. 7B. Design data forthe filter stack of the second exemplary optical filter are listed inFIG. 8B. The filter stack includes 25 layers, the AR coating includes 5layers, and the total coating thickness is about 3.3 μm.

Transmission spectra 800 and 801 at incidence angles of 0° and 20°,respectively, for the second exemplary optical filter are plotted inFIG. 8C. The second exemplary optical filter has a transmittance level,within the passband, of greater than 90%, and a blocking level, outsideof the passband, of greater than OD2 over a wavelength range of 400 nmto 1100 nm. The passband has a center wavelength of about 829 nm and aFWHM of about 29.6 nm at an incidence angle of 0°. With change inincidence angle from 0° to 20°, the center wavelength of the passbandshifts by about −7.8 nm.

Similarly to the first exemplary optical filter of FIG. 7, the secondexemplary optical filter of FIG. 8, advantageously, includes fewerlayers, has a smaller total coating thickness, and has a lowercenter-wavelength shift with change in incidence angle than the secondconventional optical filter of FIG. 2.

The second exemplary optical filter may also be compared to an analogousoptical filter including an Si/SiO₂ filter stack instead of an Si:H/SiO₂filter stack. Transmission spectra 810 and 811 at incidence angles of 0°and 20°, respectively, for this optical filter are plotted in FIG. 8D.The transmittance level within the passband of this optical filter istoo low to be useful.

With reference to FIG. 9, a third exemplary optical filter is a narrowbandpass filter that is designed to transmit light over a wavelengthrange of 845 nm to 865 nm over an incidence angle range of 0° to 40°.The third exemplary optical filter of FIG. 9 is comparable to the thirdconventional optical filter of FIG. 3. Design data for the AR coating ofthe third exemplary optical filter, which is the same as that of thefirst exemplary optical filter, are listed in FIG. 7B. Design data forthe filter stack of the third exemplary optical filter are listed inFIG. 9A. The filter stack includes 29 layers, the AR coating includes 5layers, and the total coating thickness is about 4.8 μm.

Transmission spectra 900 and 901 at incidence angles of 0° and 40°,respectively, for the third exemplary optical filter are plotted in FIG.9B. The third exemplary optical filter of FIG. 9 has substantially thesame passband width as the third conventional optical filter of FIG. 3,but has a slightly lower transmittance level within the passband.Advantageously, however, the third exemplary optical filter acceptslight over a considerably larger incidence angle range of 0° to 40° thanthe third conventional optical filter, which accepts light over anincidence angle range of only 0° to 24°. In other words, the thirdexemplary optical filter has a significantly wider incidence-angleacceptance range. Also advantageously, the third exemplary opticalfilter includes fewer layers and has a smaller total coating thickness,about one fifth of the total coating thickness of the third conventionaloptical filter.

As mentioned heretofore, the optical filter of the present invention isparticularly useful when it forms part of a sensor system, such as aproximity sensor system, a 3D imaging system, or a gesture-recognitionsystem. With reference to FIG. 10, a typical sensor system 1000 includesa light source 1010, an optical filter 1020 according to the presentinvention, and a sensor 1030. Note that other elements commonly includedin a sensor system, such as optics, are omitted for simplicity ofillustration.

The light source 1010 emits light at an emission wavelength in awavelength range of 800 nm to 1100 nm. Typically, the light source 1010emits modulated light, e.g., light pulses. Preferably, the light source1010 is a light-emitting diode (LED), an LED array, a laser diode, or alaser diode array. The light source 1010 emits light towards a target1040, which reflects the emitted light back towards the sensor system1000. When the sensor system 1000 is a gesture-recognition system, thetarget 1040 is a user of the gesture-recognition system.

The optical filter 1020 is disposed to receive the emitted light afterreflection by the target 1040. The optical filter 1020 has a passbandincluding the emission wavelength and at least partially overlappingwith the wavelength range of 800 nm to 1100 nm. Typically, the opticalfilter 1020 is a bandpass filter, preferably, a narrow bandpass filter,as described heretofore. The optical filter 1020 transmits the emittedlight from the light source 1010, while substantially blocking ambientlight. In short, the optical filter 1020 receives the emitted light fromthe light source 1010, after reflection by the target 1040, andtransmits the emitted light to the sensor 1030.

The sensor 1030 is disposed to receive the emitted light aftertransmission by the optical filter 1020, i.e., the sensor 1030 isdisposed behind the optical filter 1020. In some instances, the opticalfilter 1020 is formed directly on the sensor 1030 and, thus, disposed onthe sensor 1030. For example, the optical filter 1020 may be coated andpatterned, e.g., by photolithography, on sensors, e.g., proximitysensors, in wafer level processing (WLP).

When the sensor system 1000 is a proximity sensor system, the sensor1030 is a proximity sensor, which detects the emitted light to sense aproximity of the target 1040, according to methods known in the art.When the sensor system 1000 is a 3D-imaging system or agesture-recognition system, the sensor 1030 is a 3D image sensor, e.g.,a charge-coupled device (CCD) chip or a complementary metal oxidesemiconductor (CMOS) chip, which detects the emitted light to provide a3D image of the target 1040, which, in some instances, is the user.Typically, the 3D image sensor converts the optical information into anelectrical signal for processing by a processing system, e.g., anapplication-specific integrated circuit (ASIC) chip or a digital signalprocessor (DSP) chip, according to methods known in the art. Forexample, when the sensor system 1000 is a gesture-recognition system,the processing system processes the 3D image of the user to recognize agesture of the user.

Of course, numerous other embodiments may be envisaged without departingfrom the spirit and scope of the invention.

We claim:
 1. An optical filter, comprising: a filter stack comprising: aplurality of thin film hydrogenated silicon layers, wherein theplurality of thin film hydrogenated silicon layers have a refractiveindex of greater than 3 over a wavelength range of 800 nm to 1100 nm;and a plurality of thin film dielectric lower-refractive-index layers,wherein the plurality of thin film dielectric lower-refractive-indexlayers have a refractive index of less than 3 over the wavelength rangeof 800 nm to 1100 nm, and wherein the plurality of thin film dielectriclower-refractive-index layers alternate, in a one-to-one ratio, with theplurality of thin film hydrogenated silicon layers; and wherein theoptical filter has exhibits interference that creates a passband atleast partially overlapping with the wavelength range of 800 nm to 1100nm, wherein the passband has a center wavelength that shifts by lessthan 20 nm in magnitude with a change in an incidence angle between 0°to 30°.
 2. The optical filter of claim 1, wherein the plurality of thinfilm hydrogenated silicon layers each have a refractive index of greaterthan 3.6 at a wavelength of 830 nm.
 3. The optical filter of claim 1,wherein the plurality of thin film hydrogenated silicon layers each havea refractive index of greater than 3.5 over the wavelength range of 800nm to 1100 nm.
 4. The optical filter of claim 1, wherein the pluralityof thin film hydrogenated silicon layers each have a refractive index ofgreater than 3.6 over the wavelength range of 800 nm to 1100 nm.
 5. Theoptical filter of claim 1, wherein the plurality of thin film dielectriclower-refractive-index layers each have a refractive index of less than2.5 over the wavelength range of 800 nm to 1100 nm.
 6. The opticalfilter of claim 1, wherein the plurality of thin film dielectriclower-refractive-index layers each have a refractive index of less than2 over the wavelength range of 800 nm to 1100 nm.
 7. The optical filterof claim 1, wherein the passband has a center wavelength that shifts byless than 15 nm in magnitude with a change in the incidence anglebetween 0° to 30°.
 8. The optical filter of claim 1, wherein thepassband has a center wavelength that shifts by about 12.2 nm inmagnitude with a change in the incidence angle between 0° to 30°.
 9. Theoptical filter of claim 1, wherein the plurality of thin film dielectriclower-refractive-index layers are each composed of silicon dioxide(SiO₂), aluminum oxide (Al₂O₃), titanium dioxide (TiO₂), niobiumpentoxide (Nb₂O₅), tantalum pentoxide (Ta₂O₅), or a mixture thereof. 10.The optical filter of claim 1, wherein the optical filter has atransmittance level, within the passband, of greater than 90% over thewavelength range of 800 nm to 1100 nm.
 11. The optical filter of claim1, wherein the optical filter has a blocking level, outside of thepassband, of greater than OD2 over a wavelength range of 400 nm to 1100nm.
 12. The optical filter of claim 1, wherein the optical filter is along-wavelength-pass edge filter, and wherein the passband has an edgewavelength in the wavelength range of 800 nm to 1100 nm.
 13. The opticalfilter of claim 1, wherein the optical filter is a bandpass filter, andwherein the passband has a center wavelength in the wavelength range of800 nm to 1100 nm.
 14. The optical filter of claim 1, wherein thepassband has a full width at half maximum (FWHM) of less than 50 nm. 15.The optical filter of claim 1, further comprising: a substrate, whereinthe filter stack is disposed on a first surface of the substrate and acoating is disposed on a second surface of the substrate opposite thefirst surface.
 16. The optical filter of claim 15, wherein the coatingis an antireflective (AR) coating.
 17. The optical filter of claim 1,wherein the optical filter has a total coating thickness of less than 10μm.
 18. The optical filter of claim 1, forming part of a sensor systemincluding a light source for emitting light at an emission wavelengthwithin the passband of the optical filter, and a sensor for detectingthe emitted light, wherein the optical filter receives the emitted lightfrom the light source and transmits the emitted light to the sensor. 19.The optical filter of claim 1, wherein the plurality of thin filmhydrogenated silicon layers are each deposited by pulsed direct current(DC) sputtering.
 20. The optical filter of claim 1, wherein theplurality of thin film hydrogenated silicon layers are associated withan extinction coefficient of approximately 0.00055 at 800 nm.
 21. Theoptical filter of claim 1, wherein the plurality of thin filmhydrogenated silicon layers are associated with an extinctioncoefficient of less than 0.0003 at 800 nm.
 22. A sensor system,comprising: an optical filter, exhibiting interference that creates apassband including an emission wavelength and at least partiallyoverlapping with a wavelength range of 800 nm to 1100 nm, being disposedto receive emitted light and transmit the emitted light, wherein theemitted light is emitted, from a light source, at the emissionwavelength in the wavelength range of 800 nm to 1100 nm, and wherein theoptical filter includes a filter stack including: a plurality of thinfilm hydrogenated silicon layers, wherein the plurality of thin filmhydrogenated silicon layers each have a refractive index of greater than3 over the wavelength range of 800 nm to 1100 nm; and a plurality ofthin film dielectric lower-refractive-index layers, wherein theplurality of thin film dielectric lower-refractive-index layers eachhave a refractive index of less than 3 over the wavelength range of 800nm to 1100 nm, and wherein thin film dielectric lower-refractive-indexlayers, of the plurality of thin film dielectric lower-refractive-indexlayers, alternate, in a one-to-one ration, with thin film hydrogenatedsilicon layers of the plurality of thin film hydrogenated siliconlayers, wherein the passband has a center wavelength that shifts by lessthan 20 nm in magnitude with a change in an incidence angle between 0°to 30°.
 23. The sensor system of claim 22, wherein the optical filter isdisposed directly on a sensor.
 24. The sensor system of claim 22,further comprising: a sensor, wherein the sensor system is a proximitysensor system, the emitted light is directed toward a target, theoptical filter is disposed to receive the emitted light after reflectionby the target, and the sensor is a proximity sensor for detecting theemitted light and sensing a proximity of the target.
 25. The sensorsystem of claim 22, further comprising: a sensor, wherein the sensorsystem is a three-dimensional (3D) imaging system, the emitted light isdirected toward a target, the optical filter is disposed to receive theemitted light after reflection by the target, and the sensor is a 3Dimage sensor for detecting the emitted light and providing a 3D image ofthe target.
 26. The sensor system of claim 25, wherein the 3D imagingsystem is a gesture-recognition system, the target is a user of thegesture-recognition system, and the sensor system further includes: aprocessing system for processing a 3D image of the user to recognize agesture of the user.
 27. The sensor system of claim 22, furthercomprising: a sensor, wherein the sensor is a charge-coupled device(CCD) chip or a complementary metal oxide semiconductor (CMOS) chip. 28.The sensor system of claim 22, further comprising: a sensor, wherein theoptical filter is formed directly on the sensor by using wafer-levelprocessing.
 29. The sensor system of claim 22, wherein the opticalfilter further includes: a substrate, wherein the filter stack isdisposed on a first surface of the substrate; and an antireflective (AR)coating is disposed on a second surface of the substrate opposite thefirst surface.
 30. A system, comprising: an optical filter, exhibitsinterference that creates a passband including an emission wavelengthand at least partially overlapping with a wavelength range of 800 nm to1100 nm; wherein the optical filter includes a filter stack including: aplurality of thin film hydrogenated silicon layers, wherein theplurality of thin film hydrogenated silicon layers have a refractiveindex of greater than 3.5 over a wavelength range of 800 nm to 1100 nm;and a plurality of thin film dielectric lower-refractive-index layers,wherein the plurality of thin film dielectric lower-refractive-indexlayers have a refractive index of less than 3 over the wavelength rangeof 800 nm to 1100 nm, and wherein thin film dielectriclower-refractive-index layers, of the plurality of thin film dielectriclower-refractive-index layers, alternate, in a one-to-one ratio, withthin film hydrogenated silicon layers of the plurality of thin filmhydrogenated silicon layers, and wherein the passband has a centerwavelength that shifts by less than 20 nm in magnitude with a change inan incidence angle between 0° to 30°.
 31. The system of claim 30,wherein the passband has a full width at half maximum of less than 50nm.
 32. The system of claim 30, further comprising: a substrate, whereinthe filter stack is disposed on a first surface of the substrate and acoating is disposed on a second surface of the substrate opposite thefirst surface.